1. Field of the Invention
The invention relates to the field of MOS digital-to-analog converters and circuits.
2. Prior Art
Numerous digital-to-analog converters are known in the art. These converters are typically fabricated with a plurality of precision resistors and often employ bipolar devices for switching and logic functions. Metal-oxide-semiconductor (MOS) technology has not been particularly useful for fabricating digital-to-analog converters. Typical MOS processing variations make it difficult to fabricate uniform resistor strings or chains; masking misalignment adds to these problems. With MOS technology it is also difficult to fabricate an "on chip" stable reference voltage source for the resistor strings. Generally in the prior art, MOS converters use externally generated reference potentials.
One MOS digital-to-analog converter employs a multi-tap resistor (string) which is formed by diffused substrate regions. A tree-like logic array, controlled by the input digital signal, is used to select the appropriate tap on the resistor string. (This converter is discussed in more detail in conjunction with FIG. 1.) While this converter has the advantage of providing a monotonic output, that is, the output analog signal is always larger for a larger input digital signal, it has numerous disadvantages. First, in its practical layout on a substrate, a significant amount of substrate area is required. The resistor string is spread over considerable substrate area (when compared to the invented converter). This decreases the uniformity of the resistors due to MOS processing variations which in turn effects the linearity of the converter. Another disadvantage of this converter is that a long path, through many switches, exists between the selected tap and the output of the converter. Thus this prior art converter has a high output impedance.
As will be seen, the disclosed digital-to-analog converter also provides a monotonic output, however, without many of the disadvantage of the prior MOS converters. The resistor strings employed in the invented converter may be closely formed on a substrate, thereby minimizing the effects of processing variations. Moreover, through a unique layout, the effects of masking misalignments are substantially reduced. The output analog signal only passes through two switching means, thus the disclosed converter has a lower output impedance.